J. Schlosser

Critical heat flux analysis and R&D for the design of the ITER divertor

The vertical target and dump target of the ITER divertor have to be designed for high heat fluxes (up to 20 MW:m 2 over :10 s). Accommodation of such high heat fluxes gives rise to several issues, including the critical heat flux (CHF) margin which is a key requirement influencing the choice of cooling channel geometry and coolant conditions. An R&D programme was evolved to address the overall CHF issue and to help focus the design. It involved participation of the four ITER home teams and has been very successful in substantially expanding the CHF data base for one-sided heating and in providing more accurate experimental measurements of pressure drop (and derived correlations) for these geometries. This paper describes the major R&D results and the design analysis performed in converging on a choice of reference configuration and parameters which resulted in a CHF margin of : 1.4 or more for all divertor components.

Heat flux deposition on plasma-facing components using a convective model with ripple and Shafranov shift

Abstract A heat flux deposition code is a very useful tool for the design of plasma-facing components. The classical cosine model representative of the convective parallel heat flux was coded within the standard finite element code of the CEA Castem 2000. Two perturbations to the idealised magnetic surfaces were taken into account: the Shafranov shift and the ripple of the toroidal magnetic field. The calculation scheme – named Tokaflu – was confronted to previous computations on bottom modular limiters and to experiments on the inner first wall. The code was used to optimise the shape of future plasma-facing components in Tore Supra designed for the CIEL project, namely the toroidal pumped limiter and its neutralisers. Developments are under way to include the shadowing of the components, other power deposition schemes and possibly later plasma sections other than circular.

Experimental optimisation of a hypervapotron® concept for ITER plasma facing components

Abstract Historically developed for cooling of klystron electronic tubes, metallic hypervapotron® prototypes with different width were manufactured for high heat flux plasma facing components (PFC) applications. They were critical heat flux (CHF) tested on the European 200 kW electron beam facility (FE200), the 54 measured values have shown their good performances—up to 25–30 MW/m2 at low axial velocities (2–6 m/s)—interesting for ITER divertor dome and vertical target design. An important conclusion is that CHF decreases when the width increases.

Experience feedback from high heat flux component manufacturing for Tore Supra

Abstract Tore Supra is involved in flat tile carbon armoured plasma facing components (PFCs) since 1985. In 1997, a third generation of components, based on the original concept developed with Plansee Company, called active metal casting (AMC®), has been launched. Since 1998, 660 elementary components for the toroidal pump limiter (TPL) are in production. The route of the manufacture is rather complex and many controls were requested all along the fabrication to insure a high reliability of the elements. One of the main controls is the final infrared (IR) test allowing to determine the quality of the bonding between the carbon fibre composite (CFC) tiles and the heat sink made of copper–chromium–zirconium alloy (CuCrZr). Although results for the first batch of elements were as expected (less than 5% rejected at the final test), unexpected defects appeared with the followings batches. Investigations on the fabrication processes underlined the importance of having a better heat treatment of the pieces in copper alloy (CuCrZr), however this was not sufficient to completely explain the observed defects.

Non-Destructive Testing of Divertor Components

This task within the EU RD (2) blind non-destructive round robin test of the prototype; (3) HHF test in FE200 electron beam (EB) facility; (4) post-fatigue blind non-destructive round robin test; (5) destructive examination. The general final conclusion was that the NDT techniques can reliably detect and locate defects having dimensions well below those, which could impair the thermal fatigue lifetime.

Dimensional analysis of critical heat flux in subcooled water flow under one-side heating conditions for fusion application

Critical heat flux (CHF) experiments have been carried out to design a high performance cooling device with pressurized subcooled water flow under one-side heating conditions for plasma facing components. Swirl, screw and hypervapotron tubes are the most efficient geometries to remove high incident heat fluxes. A dimensional analysis shows that CHF phenomenon can be featured by dimensionless groups that are the critical Boiling, the Eckert and the Reynolds numbers, the mass enthalpic quality and the ratio between liquid and vapor densities. CHF for uniformly heated smooth tubes, the maximum wall heat flux for smooth and for swirl tubes under one-side heating conditions are predicted with a reasonable accuracy by correlating these five dimensionless groups. An enhancement factor featuring the difference between one-side and uniform heating is also introduced. CHF occurrence under one-side heating is characterized by a normalized temperature, ratio between the maximal wall temperature and the onset of nucleate boiling temperature, which only depends on the pressure. According to finite element calculations performed for smooth and swirl tubes, temperature and heat flux are distributed along the inner wall of the cooling channel under one-side heating conditions. These distributions are characterized by two dimensionless numbers for each geometry: a peaking factor, which is the ratio between the maximal heat flux at the inner wall of the cooling channel and the incident heat flux, and a full width angle at half maximum of wall heat flux at the inner wall of the cooling channel. This study draws the dimensionless groups featuring CHF phenomenon not only under uniform heating, but also under one-side heating, which involve additional normalized parameters.

Design, fabrication and testing of an improved high heat flux element, experience feedback on steady state plasma facing components in Tore Supra

Abstract Actively cooled plasma facing components (PFC) have been developed and used in Tore Supra since 1985. One of the main technological problem is due to the expansion mismatch between graphite armour and metallic heat sink material. A first technology used graphite tiles with or without a reinforcement and a compliant layer, brazed with titanium copper–silver (TiCuAg) alloy. The next technology used carbon fiber material (CFC) tiles with a 2 mm pure copper compliant layer, since the good mechanical strength of the CFC allowed the reinforcement layer to be suppressed. No destructive inspection during the manufacturing procedure was found to be essential to insure a good reliability of the elements. A recent technology was developed for the new actively cooled toroidal pump limiter of Tore Supra (designed to remove up to 15 MW during 1000 s with incident heat flux up to 10 MW m −2 ). This new technology uses an active metal casting (AMC) of copper onto CFC tiles whose surface is prepared by a laser treatment. Copper compliant layer is then electron-beam welded to the heat sink. This technology is silver free and could be ITER relevant. The first bonding is X-ray controlled and the second one ultrasonic (US) tested. Finally all the elements are controlled by an infrared imaging technique during a hot water test.

European achievements for ITER high heat flux components

This paper summarises the main activities carried out by the EU Home Team to develop suitable solutions for the ITER high heat flux components, namely the divertor, the baffle and the limiter. The available results demonstrate that the EU have the capability to manufacture high heat flux components with carbon fibre reinforced carbon, tungsten and beryllium armours which all exceed the ITER design requirements.

TORE SUPRA experience of copper chromium zirconium electron beam welding

The specification of TORE SUPRA to perform quasi steady state plasma operation has induced several R&D studies on the actively cooled structures of the plasma facing components and the associated assembling processes of the materials. Various industrial copper alloys have been characterized and tested to select the most optimized grade for EB welding. Welding samples of representative geometry were also analysed.

In-service experience feedback of the Tore Supra actively cooled inner first wall

Abstract Over 12 000 plasma shots (some of them with up to 8 MW of additional power and some as long as 60 s) have been achieved in Tore Supra, with a significant number of them limited by the inner first wall. This actively water-cooled wall is covered with brazed graphite tiles. High power, high energy experiments have shown that reliability of the graphite tile-heat sink joint and accurate alignment of the wall are needed. This paper summarizes the experience gained with this component, and the developments in progress to improve the performance of such an inner first wall.